When a video starts loading, this script, startet in a frame looks into it and determines with wich resolution it was created. Then it notifies You in the upper left corner of the video about width and height separated by an "x". At the same time it informs the same script in the main page about the size so it can be resized to the native video size plus some extra space at the bottom.
RAPIDVIDEO tells You the available sizes at the bottom of the video, but only the height followed by "p". Here can You choose your preferred size, if its there. If not, bad luck.
If the video is to small, you can zoom in (e.g. with the scroll wheel of your mouse while holding the STRG-key) and watch how the video gets blurry, or just use the full screen mode. If this works for You, please consider creating a user style.
There are frequently changes at KissAnime. If something stopped to work, please report. Of course, you can report if something actually works also.